Rare-earth elements play an important role in modern technologies including, but not limited to, high-strength magnets and energy-efficient fluorescent lighting. The rare earths (REs) typically encompass 15 members of the 4f row (lanthanides, Z=57-71) and a second-row transition metal, yttrium (Z=39). The rare earths may also be considered to encompass the actinide elements (Z=89-103). Many of the rare earths occur in nature except for man-made promethium (Z=59) and several of the actinides. The lanthanides and yttrium are all thermodynamically stable trivalent ions in solution and the solid state. In addition, a phenomenon known as the lanthanide contraction is observed. This phenomenon displays an incremental decrease in the radius of the RE3+ ion from La3+ to Lu3+. As a result of the stable 3+ oxidation state and the small changes in ionic radii between adjacent lanthanides and actinides, separating rare earths is challenging. In addition to lanthanides and actinides having such similar solution chemistries, the rare earths also require their ores to be processed, which has a number of industrial challenges.
The demand on industry to provide an accessible supply of these materials is ever-increasing as rare earths continue to play a more important role in applications related to modern technology. As of 2011, the majority of world rare-earth element production is in China (95%), with Australia (2%) and India (2.5%) contributing substantially less, and Brazil and Malaysia contributing the remainder (0.47%) (e.g., Gschneidner Jr., Mater. Matters (Aldrich) 2011, 6, 32-37). In China and the U.S., bastnaesite is the mineral of most interest for rare-earth recovery. Bastnaesite, a rare-earth carbonate fluoride (RECO3F) mineral, is approximately 7-10% rare-earth oxide (REO) consisting mostly of the lighter elements (ca. 98% La—Nd) (e.g., J. B. Hedrick, “Rare Earths,” in Metals and Minerals, U.S. Geological Survey Minerals Yearbook 2001, vol. I, 61.1-61.17).
Historically, bastnaesite has undergone a series of physicochemical processes to produce a commercial product. Considering the flow sheet for bastnaesite employed by Molycorp (F. F. Aplan, The processing of rare-earth minerals. The Minerals, Metals and Materials Society, Warrendale, Pa., USA, 1988), the following generalized process may be written:
Bastnaesite ore (7% REO)→crushing/grinding→conditioning→series of flotation steps→leaching→calciner→bastnaesite→concentrate (90% REO)→separation plant
After this beneficiation process, chemical treatment of either the crude ore or the bastnaesite concentrate may take place, e.g., P. R. Kruesi, G. Duker, Min. Met. Mater. S 1965, 17, 847. The process involves leaching bastnaesite with hydrochloric acid, treating the resulting rare-earth fluorides (REF3) with sodium hydroxide, and finally, solubilizing the rare-earth hydrolysis product with hydrochloric acid. In general, this process employs the following steps:3RECO3F+9HCl→REF3+2RECl3+3HCl+3H2O+3CO2 REF3+3NaOH→RE(OH)3+3NaFRE(OH)3+3HCl→RECl3+3H2O
However, this chemical treatment process requires heating of the ore and industrial solutions to roughly 95° C. for 4 hours at different stages and the consumption of 2.5 kg HCl/kg of RE2O3 and 0.73 kg NaOH/kg REO feed to achieve the final product. Considering the significant expenditures in energy, time, and cost for current rare earth extraction methods, there would be a significant benefit in a simplified and less costly process for achieving the same end.